There are previously known apparatus and methods for employing arrays of cathode-ray tube ("CRT") displays in "video walls" and signage applications. Multiscreen displays employ an abutted array of substantially identical display devices that each display a subdivided portion of a total image such that together they display the total image. Because multiscreen displays require that each of the display devices be perceived as part of a single large display device, it is important to make the boundaries between adjacent display devices appear as inconspicuous as possible.
Unfortunately, the human eye is very sensitive to boundary discontinuities, making a "seamless" multiscreen display very difficult to produce. This is especially true for arrays of CRT displays because of their nonrectangular shapes, curved faceplates, and nondisplayable borders. Of course, it is also important to electronically subdivide the video so that each display seamlessly displays only its portion of the total image.
Nevertheless, prior workers have successfully overcome many of these problems by employing arrays of projection CRT displays coupled to intelligent video display controllers. An exemplary multiscreen display system employs an array of PROCUBE AC CRT projection displays coupled to a PICBLOC 3 display controller, both of which are manufactured by Electrosonic Systems, Inc. of Minneapolis, Minn.
Moreover, there is also a need for luminance uniformity and color balance among the displays in an array because the human eye also easily perceives luminance and color differences between adjacent displays. Therefore, the above-described system employs a manually operated luminance and color balancing system and an optional external image sensor with which the luminance and color of each CRT projection display may be manually set to match a predetermined factory standard.
Indeed, even color and luminance shading variations in an individual display can degrade the total image displayed on a multiscreen display. For example, U.S. Pat. No. 5,396,257, issued May 7, 1995, for MULTISCREEN DISPLAY APPARATUS describes a CRT-based color and luminance sampling and correction system that corrects for shading differences in each display. The system employs an intelligent controller to coordinate luminance and color samples from each display and store correction data in lookup tables that are associated with each display, but which also account for the color and luminance of the overall multiscreen display.
Despite this prior work, CRT-based multiscreen displays still have image stability, weight, form factor, and image matching problems. Clearly, a digitally addressed, compact, lightweight display would solve many of these problems. Liquid crystal projection displays have evolved to a point where many of the above problems can be solved. For example, "A 750-TV-Line-Resolution Projector Using 1.5-Megapixel a-Si TFT LC Modules," Takeuchi et al., Society for Information Display, SID 91 DIGEST, pp. 415-418, describes such a display. Unfortunately, liquid crystal displays have not been readily accepted for use in multiscreen display applications because they have projection lamp-induced luminance variations and liquid crystal display transfer function variations that make color balancing difficult. Referring to FIG. 1A, a typical transfer function of a liquid crystal display ("LCD") is an S-curve shaped response 2 that produces a relative LCD luminance that is nonlinear with respect to its relative drive voltage. Assume that the "gain" of the LCD is reduced to 80 percent of its maximum value by attenuating its maximum drive voltage. Its reduced gain response is the portion of S-curve shaped response 2 that is bounded by dashed lines 4, and is clearly not an S-curve. The portion of S-curve shaped response 2 bounded by dashed lines 4 is shown in FIG. 1B as reduced gain response 6, and is overlayed therein with S-curve shaped response 2. Now, if two LCDs each having S-curve shaped response 2 are employed in a color LCD, and one of them is operating at the reduced gain to achieve a predetermined color balance, the overall response of the color LCD is as shown in FIG. 1B. One LCD operates with S-curve shaped response 2, and the other operates with reduced gain response 6. Comparing the two responses shows that a 1.0 relative drive voltage produces maximum output from each LCD (remember that the reduced gain LCD provides only 0.8 the relative luminance of the other LCD). Of course, at 0.0 relative drive voltage, both LCDs produce zero relative luminance. However, at an intermediate 0.7 relative drive voltage, the LCD with response 2 provides 0.82 relative luminance, whereas the LCD with reduced gain response 6 provides only 0.44 relative luminance. Indeed, most intermediate values of drive voltage will produce unbalanced amounts of relative luminance. In a color LCD having two or more LCDs, this unbalance causes unacceptable color shifts as the relative drive voltage changes. Moreover, neither S-curve response 2 nor gain corrected response 6 matches the well-known gamma curve response employed by conventional video signal sources to match the nonlinear luminance response of the human eye.
What is needed, therefore, is a color balance and luminance correction apparatus and method for liquid crystal projection displays that renders them suitable for use in color and/or multiscreen display applications.